We have to explain our choice of nomenclature. Basic concepts of
physics say space contains homogeneous zero-point energy, and
maybe also energy that
is homogeneous or nearly so in other forms, real or effective (as
from counter terms in the gravity physics, which make the net
energy density cosmologically acceptable). In the literature this
near homogeneous energy has been termed the vacuum energy, the
sum of vacuum energy and quintessence
(Caldwell, Davé, and
Steinhardt, 1998),
and the dark energy
(Turner, 1999).
We have adopted the last term, and we will refer to the dark energy density
that manifests itself as an effective version of
Einstein's cosmological constant, but one that may vary slowly
with time and position. 6

Our subject involves two quite different traditions, in physics
and astronomy. Each has familiar notation, and familiar ideas
that may be "in the air" but not in the recent literature.
Our attempt to take account of these traditions commences with
the summary in Sec. II of the basic notation
with brief
explanations. We expect readers will find some of these concepts
trivial and others of some use, and that the useful parts will
be different for different readers.

We offer in Sec. III our reading of the
history of ideas on
and its
generalization to dark energy. This is a
fascinating and we think edifying illustration of how science may
advance in unexpected directions. It is relevant to an
understanding of the present state of research in cosmology,
because traditions
inform opinions, and people have had mixed feelings about
ever since
Einstein (1917)
introduced it 85 years ago.
The concept never entirely dropped out of sight in cosmology
because a series of observations hinted at its presence, and
because to some cosmologists
fits the
formalism too well to be ignored. The search for the physics of the
vacuum, and its possible relation to
, has a long
history too. Despite the common and strong suspicion that
must be
negligibly small, because any other acceptable value is absurd,
all this history has made the community well prepared for the
recent observational developments that argue for the detection of
.

Our approach in Sec. IV to the discussion of
the evidence for
detection of ,
from the cosmological tests, also requires
explanation. One occasionally reads that the tests will show us
how the world ends. That certainly seems interesting, but it is
not the main point: why should we trust an extrapolation into the
indefinite future of a theory we can at best show is a good
approximation to reality? 7
As we remarked in Sec. I.A, the purpose of
the tests is to check the approximation to reality, by checking the
physics and astronomy of the standard
relativistic cosmological model, along with any viable alternatives
that may be discovered. We take our task to be to identify the
aspects of the standard theory that enter the interpretation of the
measurements and thus are or may be empirically checked or measured.

6 The dark energy should of course be
distinguished from a
hypothetical gas of particles with velocity dispersion large
enough that the distribution is close to homogeneous.
Back.

7 Observations may now have detected
, at a characteristic
energy scale of a millielectronvolt (Eq. [47]).
We have no guarantee that there is not an even lower energy scale;
such a scale could first become apparent through the cosmological tests.
Back.